Is this an intrusive or an extrusive igneous rock?

From this view the amazing structure of rocks that make up Devil's Tower doesn't really indicate whether the structure formed slowly or quickly. A close up view would show small crystals in a mafic rock, indicating a rapid cooling from a basalt lava. Cooling was slow enough that the hexagonal "posts" could form.

Igneous Rock Classification

Igneous rocks are first classified by their composition, from felsic to ultramafic. The characteristics and example minerals in each type are included in Tablebelow.

Properties of Igneous Rock Compositions

Composition

Color

Density

Minerals

Felsic

Light

Low

Quartz, orthoclase feldspar

Intermediate

Intermediate

Intermediate

Plagioclase feldspar, biotite, amphibole

Mafic

Dark

High

Olivine, pyroxene

Ultramafic

Very dark

Very high

Olivine

Second to composition in igneous rock classification is texture. Texture indicates how the magma that formed the rock cooled.

Silica Composition and Texture of Major Igneous Rocks

Type

Amount of Silica

Extrusive

Intrusive

Ultramafic

<45%

Komatiite

Peridotite

Mafic

45-52%

Basalt

Gabbro

Intermediate

52-63%

Andesite

Diorite

Intermediate-Felsic

63-69%

Dacite

Granodiorite

Felsic

>69% SiO2

Rhyolite

Granite

Some of the rocks in Tableabove were pictured earlier in previous concepts about rocks. Look back at them and, using what you know about the size of crystals in extrusive and intrusive rocks and the composition of felsic and mafic rocks, identify the rocks in the photos in Figurebelow:

These are photos of A) rhyolite, B) gabbro, C) peridotite, and D) komatiite.

Summary

Composition is the first criteria on which to classify igneous rocks, with categories from felsic to ultramafic; color is a first order indicator of composition.

Texture is the second criteria for classifying igneous rocks because texture indicates how a rock cooled.

Igneous rocks are categorized in pairs with the same composition but different textures: gabbro-basalt, diorite-andesite, and granite-rhyolite.

Review

Describe the formation of the igneous rock pair gabbro-basalt. What makes the rocks the same and what makes them different?